1. Field of the Invention
The present invention relates to a metal oxide semiconductor (MOS) integrated circuit apparatus, and more particularly, relates to the protection of a MOS device.
2. Description of the Prior Art
To improve the integration of a metal oxide semiconductor (MOS) integrated circuit, it is inevitable that the structure of a MOS transistor is minute. On the contrary, if the structure of the MOS transistor is minute, the channel length decreases, so that a hot electron effect remarkably decrease the reliability of the MOS transistor. For this reason, in recent years, a lightly doped drain (LDD) structure has been used instead of the conventional single drain structure.
With the LDD structure, compared to the single drain structure which is of high density, the extent of the depletion layer is large, the electric field is weak, and the substrate current and the gate current decrease, so that the reliability increases.
In the LDD structure where the oxide film of the gate is thin, however, the electrostatic breakage readily occurs. Thus, in the CMOS integrated circuit apparatus, the electrostatic breakage withstanding voltage of the input and output terminals decreases as the structure of the device becomes minute. For this reason, a protecting device is provided at the signal input and output terminals so that no high voltage is directly applied to the protecting device and to the gate oxide film of the CMOS device provided inside.
The number of devices connected to the power terminal acceleratingly increases as the integration advances, so that the withstanding voltage of the power terminal against static electricity is decreased by errors caused in checking the circuits in designing and in checking the pattern layout. In the CMOS integrated circuit, a parasitic transistor is inevitably generated. Since the parasitic transistor constitutes a thyristor, once it is activated, the activated condition is maintained, so that the operation of the CMOS device is damaged. The condition where the activated condition of the parasitic transistor is maintained is called a latch up.